Many artists use acrylic paints which air dry as water leaves the paint. As water evaporates or is absorbed by the substrate to which the paint is applied, acrylic polymer particles coalesce into a continuous adhesive film. Within a relatively short period of time, approximately 10 to 15 minutes, a skin forms over the outer surface of the paint film. At this stage the paint film is said to be “dry to the touch”. The thicker films continue to dry over time, the length of time varying with the thickness of the film. For very thin films this may be a few minutes while films half a centimetre in thickness or more may take quite some time longer.
Once the acrylic particles coalesce, the film can no longer be worked. The artist is not able to change and adapt what is taking place on the canvas using traditional techniques. Although the dry film could be painted over, the “wet in wet” painting techniques are severely curtailed. The term “wet in wet” is used by persons skilled in the art to refer to painting techniques by which the artist creatively works and develops the wet paint on the substrate, adding, blending and altering the appearance over time to achieve the desired artistic result.
Temperature, humidity and air flow are environmental factors that influence the drying times of acrylic paint compositions. The time taken for a paint film to skin over can be extended somewhat by reducing air-flow, lowering temperature and increasing humidity. Manipulating such environmental factors may not always be possible or practical. Low temperatures (below about 9° C.) may interfere with coalescence of the polymer particles resulting in paint film cracking, becoming powdery or failing to adhere to the substrate. Optimum humidity may be difficult to determine and maintain.
The nature of the substrate also influences the drying time of acrylic paint compositions. An absorbent substrate will draw moisture out of the paint composition and speed drying time. Sealing the substrate to reduce absorbency or selecting a non-absorbent substrate such as metal or masonite will reduce the moisture lost through the substrate. Using a moist substrate can also reduce moisture lost through the substrate and increase humidity in the environment of the paint. By soaking the back of a canvas with water or attaching wet rags or sponges to the back of a canvas, for example, the rate of drying of the paint film can be reduced. This technique has limitations and the water may also wash impurities out of the canvas into the paint film causing discoloration.
Humectants, such as glycerin or propylene glycol may be used as paint additives to extend the time taken for a paint film to skin over. As paint additives however, humectants tend to linger in the paint film. They can remain tacky and adversely affect outer coat adhesion. Most importantly they do not prevent fast drying of thin films of paint.
The present inventor has previously found that a water based, acrylic, air dried, artists' paint composition with improved “open time” properties can be obtained by including an alkali soluble polymer and specific substituted amines into a conventional acrylic artists paint composition. The “open time” of the resulting composition could be extended provided the artist continually applied water to the paint composition. The extended “open time” provided the artist with a longer time to use the “wet in wet” painting technique. This is described in WO 2007/009161, the contents of which are incorporated herein.
However, the artist loses the opportunity to paint “wet in wet” once the paint composition has dried. The need for the artist to be vigilant in observing the wetness of the paint composition and continuously apply water to the paint composition while working “wet in wet” is time consuming, inhibitive to the artists creativity and clumsy. Further, this technique is not useful for an artist painting on a large scale where sections of the painting dry earlier than others as the artist is required to be watchful of the wetness of the paint composition over a number of different areas of the painting at any one time over a very large area. Accordingly, the inventor has recognised the need for an improvement in accessing, extending and/or controlling the “open-time” of an artists paint composition.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.